Process for the production of bimetallic material with high resistance to transcrystalline stress corrosion in a chloride environment



NOV. 18, 1969 BACKSTROM ET AL 3,479,181

PROCESS FOR THE PRODUCTION OF BIMETALLIC MATERIAL WITH HIGH RESISTANCETO TRANSCRYSTALLINE STRESS CORROSION IN A CHLORIDE ENVIRONMENT FiledFeb. 14, 1968 3 Sheets-Sheet 1 v INVENTORS. ARNE IVAR BAcKsTRZiM, STUREHENRIKSON a PER ERIK LEVIN W HDMJ T eir ATTORNEYS.

Nov. 18, 1969 A. l. BAC TROM ETAL 3,479,181

PROCESS F THE PRODUCTION BIMETALLIC MATERI WITH HIGH RES ANCE TOTRANSCRYSTALLINE STRESS COR ION IN A CHLORIDE ENVIRONMENT Filed Feb. 14,1968 3 Sheets-Sheet 2 FIG.2

INVENTORS.

ARNE IVAR BACKSTROM, STURE HENRIKSON a PER ERIK LEVIN 1 IMA PM eirATTORNEYS.

Nov. 1969 Filed Feb. 14, 1968 A. l. BACKSTROM ET ROCESS FOR THE PRODU onOF BIMETALLIC MAT A RESISTANCE TO TB SCR ALLINE STRESS R ON IN A CHLORENT ENVIRONM 3 Sheets-Sheet 5 IN VE N TORS.

STURE HENRIKSON PER ERIK LEVIN M l-3:11 m Drrw heir ATTORNEYS.

ARNE IVAR BACKS "M, v

United States Patent O 3,479,181 PROCESS FOR THE PRODUCTION OFBIMETALLIC MATERIAL WITH HIGH RESISTANCE T TRANSCRYSTALLINE STRESSCORROSION IN A CHLORIDE ENVIRONMENT Arne Ivar Backstrom, StureHenrikson, and Per Erik Levin, Avesta, Sweden, assignors to AvestaJernverks Aktiebolag, Avesta, Sweden, a Swedish joint-stock companyFiled Feb. 14, 1968, Ser. No. 705,490 Claims priority, applicationSweden, Feb. 16, 1967,

Int. 01. e226 33/02; B22f 1/00 US. Cl. 75-212 Claims ABSTRACT OF THEDISCLOSURE The normal types of austenitic chromium-nickel andchromium-nickel-molybdenum steel may, under certain conditions, beaffected by so-called transcrystalline stress corrosion.

The corrosion attacks usually take the form of cracks which rapidlyforce their way deeper and deeper into the material, often breakingthrough entirely.

Transcrystalline stress corrosion will occur under the combined effectof the following factors:

1) Tensile stresses. These may be residual, conditional on the design(resulting from welding, bending, machining, etc.) or have anoperational basis (from pressure or temperature). The sensitivity riseswith an increasing tensile stress. It is diflicult to indicate thelowest stress which causes stress corrosion, since the sensitivity isalso controlled by other factors. If the latter are severe, however, astress corresponding to half the tensile stress limit at the temperaturein question is sufiicient to cause the formation of cracks.

(2) Specific corrosion media. The most common materials causing stresscorrosion are chlorides, such as KCl, CaCl NaCl and MgCl In practicethese chlorides are present in varying contents both in natural coolingwater and in special cooling media. The sensitivity to stress corrosionincreases with increasing chloride content. It is impossible, however,to indicate any threshold values below which the respective steelqualities are insensible owing to the risk for a local concentration inpractical operation of contents no matter how low (e.g. by theevaporation of water).

(3) Temperature. The stress corrosion caused by chlorides is very rarebelow 4050 C. However, the sensitivity increases considerably withincreasing temperatures, a circumstance which is reflected by thechloride contents required to cause the stress corrosion. The chloridecontents are the absolute contents in a water solution where noconcentration takes place, thus achieved in ideal conditions duringlaboratory tests. For a steel with 18 percent chromium and 8 percentnickel and at 100- C. this content lies at about 50,000 p.p.m. (5 per-Patented Nov. 18, 1969 cent) CI, but at 150 C. and correspondingoverpressure it is as low as 10 ppm.

(4) The redox potential of the solution. During laboratory tests it hasbeen proved that the potential at which the steel sets in a chloridesolution according to the redox potential in this solution is ofdecisive importance for stress corrosion. As a matter of fact stresscorrosion seems to occur within a relatively limited potential range,about 30 mv, which is characteristic for each type of steel and ishigher the more alloyed the steel is. Below this range of potential acathodic protection of percent is obtained.

(5) Composition of the steel. Nickel is the alloying element which mosteffectively reduces the sensitivity to stress corrosion. However, about40 percent is required if this type of corrosion is entirely to beprevented. Chromium, silicon, molybdenum and copper may also increasethe corrosion resistance to a certain extent.

It would seem obvious from what has been referred to above that a highnickel content is the most effective way to reduce or prevent stresscorrosion. However, the nickel content must be maintained at a level ofabout 40 percent which, as a matter of fact, means that one cannot anylonger talk about stainless steel but must instead refer to nickelalloys since the nickel content is higher than the iron content. Asexamples of commercial nickel alloys which are resistant to stresscorrosion the following may be mentioned:

Percent Mn Fe 1 Minimum.

However, all the above alloys are expensive, mainly owing to thecomparatively high nickel price.

The object of the present invention is to arrive at a process for theproduction of bimetallic austenitic stainless steel alloys with a highresistance to transcrystalline stress corrosion in a chlorideenvironment and mainly characterized in that austenitic stainless steelpowder is plated with nickel, if desired in combination with othermetals, in layers practically coating each powder grain and where thesaid layers form corrosion-impeding barriers mainly orientated in planescoinciding with the tensile direction, after the powder material hasbeen subjected to heat treatment as well as densifying and stretching bymeans of plastic shaping, such as rolling, forging or extrusion. Thepowder material is in that manner transformed into solid material in theshape of egg. sheet metal, strips, bars or pipes. By stretching thematerial in the working process one obtains barriers relatively evenlydivided in the basic material and orientated in the tensile direction.The distance between the barriers is determined by the rate of reductionin the working process and the particle size while the thickness of thebarriers is further determined by the amount of metal added and thescope of the diffusion. The composition of the barriers depends on thecomposition of the plating, on the dilfusion within the barriers andbetween these and the basic material, and finally on the composition ofthe basic material.

The heat treatment could appropriately be conducted at a temperaturebetween 950 and 1300 C. The heat treatment may be conducted separatelyOr be coincident with the heat treatment which usually takes place priorto the plastic working.

The process makes possible considerable saving of expensive alloyingmetals, especially nickel, since only a small part by Weight is requiredof the amount which would be required if a homogeneous material With acomposition identical to the barrier material were to be produced.

The present invention is further illustrated by the following examples,reference being simultaneously made to the attached drawings, where:

FIG. 1 shows a micrograph of a type of steel according to the presentinvention, magnified 150 times,

FIG. 2 shows schematically the diffusion process in a type of steelaccording to the invention, and

FIGS. 3A and 3B show schematically the densifying process of a platedgrain.

it was compared with. However, it is difiicult numerically to access theresults. An attempt at such an assessment was however made in connectionwith the continued testing of the powder steel specimen (a) for anadditional twenty days. A powder steel specimen (d) with a compositioncorresponding to (a) but without barriers was also testedsimultaneously. Table 2 below shows this numerical comparison expressedas an index of corrosion resistance and based on the relation betweenthe time and the maximum depth of cracks. The powder steel (a) withbarriers showed a higher corrosion resistance than the steels (b) and(c). As compared with steel (b) the resistance was 5 times as high andas compared with steel 3 times as high.

TABLE 2.-TRANSCRYSTALLINE STRESS CORROSION [Testing according to Table1] Time until the Time elapsed until ap- Index of first crack Testbroken Maximum depth pearance of crack of a. corrosion Quality appearedoff alterof cracks depth of 1.5-1.6 mm. resistance (a). 11days 35 days16mm 35 days 1.4 (b) 1 day 11 days 2 mm- 0. 2 (c) 1day 11days. .1.5mm0.0 (d) 7days 13 days 1.6mm 13 days 0.3

1 Approximate value assuming a linear function.

EXAMPLES A stainless austenitic steel composed of 0.06% C, 17.0% Cr,9.3% Ni and balance Fe in powder form with a grain size of l.17:0.1 mm.was electrolytically plated with 4.3% Ni calculated on the total weightof the plated material. The powder was enclosed in a casing ofnon-alloyed steel, heated in a furnace at 1200 C. for 15 minutes andforged out to sheet metal with a thickness of 4 mm. and with a reductionof 1:5. The sheet metal was freed from the protective coating by meansof pickling in nitric acid and hydrofluoric acid.

A stress corrosion test was carried out in the following way. A testspecimen (specimen (a) in the table below) was cut out, formed to a sizeof 3 x x 150 mm., cold-bent over a mandrel with a radius of 7.5 mm. andfastened to a holder that covered a mm. portion of the specimen. Thetest specimen was immersed in a percent CaCl solution at pH 6 and 100 C.

For the sake of comparison two standard materials rolled from castingots were corrosion tested in the same way. The one steel (b) retaineda nickel content which corresponded fairly well to the basic content ofthe powder steel, while the other (e) corresponded approximately to thepowder steel as to its average content of nickel.

After completing the test the test specimens were cut in a longitudinaldirection in the places where the largest cracks had appeared, thegreatest depth of the cracks was then measured and in certain casesmicrophotographed.

The results of the test are shown in Table 1 below.

It was startling to find that the powder steel (d) with-v out barriers'had a resistance that was higher than that of steel (b) with a similaranalysis with which it has compared, or to be more specific twice ashigh.

The time elapsed until the appearance of the first crack also gives anindication of the corrosion resistance. The dispersion of the results isusually rather wide in this test and that is why the results accordingto Table 1 and Table 2 ought to be considered with a certain caution.However, it is quite obvious that steel (a) yielded better results thanthe steels it was compared with. It is surprising that even steel (d)yielded better results than steels (b) and (0).

FIG. 1 is a microphoto of the cracks in steel (a) with barriers after a15 days test in a chloride solution. It shows how from the surface thecracks have forced their way down into the material to be blocked thereby the first barrier, except in one point where the barrier was brokenthrough. The crack front has then been caught by the next barrier.

In the course of a microsecond examination the nickel contents of thebarriers varied between 20 and percent, the lower content in the thinbarriers and the higher content in the thick ones. By the heattreatment, for instance in connection with the plastic working, nickeldiffuses from the barriers out into the basic material while acorresponding diffusion of iron, chromium and the other alloyingelements present in the basic material takes place in the oppositedirection.

The diffusion process is schematically shown in FIG. 2, where theleft-hand Part I is meant to show a barrier TABLE 1.TRANSCRYSTALLINESTRESS CORROSION, U-SHAPED SPECIMENS Solution: 40 percent CaOlz, pH 6,C.

Specimen sizes: 3 x 20 x m.

Surface of specimen after- Time until the Maximum depth first crack Testbroken Completed of cracks after Qu y appeared off after- 11 days testcompleted test (a) Powder steel according to the invention: 17 per- 11days 15 days Single short (5 mm.) 6 short (5 mm.) 0.32 mm.

cent 01', 9.3+4.3=13.6 percent Ni, 0.06 percent 0. cracks. cracks. ((2)Conventional steel: 18 percent Cr, 10.2 percent 1 day 11 days Greatnumber of See preceding 2.50 mm.

Ni, 0.4 percent M0, 0.044 percent 0. cracks across the column.

entire width of the specimen. (1:) Conventional steel: 16.6 percent Cr,14.8 percent 1 day 11 days Number of 10 mm. do 1.52 mm.

Ni, 4.3 percent Mo, 0.047 percent 0. long cracks.

As is evident from the corrosion test, the powder with contiguous basicmaterial prior to diffusion and steel (a) produced far better resultsthan the specimens 75 the right-hand Part H the same material afterdiffusion.

The two upper part figures marked A (A(I) and A(II) respectively)show-on a greatly magnified scale-the same part section through twocontiguous grains 1 with intervening barrier 2. Barrier 2 has beenformed by adjoining two coated grains, the intermediate surface betweenthese grains is indicated by dash-dotted line 4. The interface betweenthe grains and the coating is marked 5. Part figure A(II) to the rightillustrates the principle of how the diffusion changes the composition.The basic material of the grains (austenitic stainless steel) haspenetrated into barrier 2 and the barrier material has penetrated intothe basic material, a condition illustrated by the extension of therespective sectioning lines. In this way interface 5 has been erased butits former position is indicated by dash-dotted lines 5. Thus, areas 3containing sectioning lines with two different inclinations consist ofmaterials with composition gradients, i.e. increasing or' decreasingcontents of the chemical elements present in the basic material and thebarrier.

The two central part figures marked B (B (I) and B(II)) as well as thetwo bottom part figures marked C (C(I) and C(11)) are meant toillustrate the composition of an arbitrary cross section through thesection shown in part figures -A(I) and A(II).

According to the embodiment of the invention shown in B(I) and B(II),austenitic stainless steel is coated with nickel. Basic material 1contains about 70 percent Fe, 20 percent Cr and percent Ni, barriermaterial 2 contains 100 percent Ni. The vertical dash-dotted and dashedlines showing the iron and chromium contents respectively in interface 5have been displaced in parallel for the sake of clarification. As amatter of fact the lines for Fe, Cr and Ni coincide in interface 5,since no diffusion has taken place when the grains were coated. Partfigure B(II) shows how the iron content decreases towards the barrierwith a lowest value of about 43 percent Fe in the centre of the barrier.The Cr content decreases in the same way and reaches a lowest value ofabout 7 percent in the centre of the barrier. Contrarywise the Nicontent increases and reaches the highest value of about 50 percent inthe centre of the barrier. As was done in part figure A(II), the formerposition of interface 5 has been indicated by dash-pointed line 5.

For the plating of the powder material not only nickel may be used, butalso nickel in combination with chromium, molybdenum, silicon, titanium,manganese and/ or copper. The total amount of plated metal may reach 1to 10 percent calculated on the powder weight preferably however 2m 6percent.

The plating of the powder material may be carried out by conventionalplating methods such as, for instance, electrolytically, via volatilemetal compounds (halides, carbonyls) and by means ofevaporationsublimation.

In case one wishes to add other metals besides nickel these may beapplied simultaneously or' one at a time. It is usually appropriate tocoat the outside of the metal grains with nickel which has a lowoxygenaffinity. This method offers considerable advantages in connectionwith the heating and handling of the powder up to the stage where thematerial is available in a compact shape and is no longer so sensitiveto oxidation. If, for instance, a powder in whose surface there is ahigher concentration of chromium is to be heated to say 1150 C., thisheating must be conducted in high vacuum in a very pure inert gasatmosphere or in hydrogen gas with an H /H O- ratio of a minimum of5-10-10 :l if there is to be no oxidation. If, on the other hand, thepowder surface has been plated with a layer of nickel, the question ofthe protective gas is very simple since the corresponding H H O-ratioonly needs to reach a minimum of 10 :1. Even nitrogen gas may be used asa protective gas or a protective gas component, since at the abovetemperature there is no risk for the nickel of nitride formation.

According to the embodiment in part figures 2C(I) and 2C(II) the grainshave first been gas chromiumplated and subsequently nickel-plated. Herethe object of the added chromium is to give the barriers a chromiumcontent approximately corresponding to the chromium content in the basicmaterial and thus give it general corrosion properties of an equalvalue.

The composition of the basic material is about '70 percent Fe, 20percent Cr and 10 percent Ni. In the process of gas chromium-plating adiffusion takes place whereby the composition of the basic material inthe surface layer is changed to about 35 percent Cr, about 57 percent Feand about 8 percent Ni. After the nickel-plating of the grains and thediffusion treatment the composition changes once more ina 'way which isanalogous to the embodiment according to part figures 2B. Thus the Fecontent decreases to about 32 percent and the Cr content to about 18percent in the centre of the barrier while the Ni content increases toabout 50 percent.

By means of this embodiment of the present invention barriers have beencreated which have a high resistance both to transcrystalline corrosionin a chloride environment and to general corrosion.

The basic material i.e. the powder material prior to plating shall be ofan austenitic stainless steel type or an acid-proof steel type with anickel content of from 5 to 15 percent and a chromium content of from 16to 25 percent. In addition the steel may contain, say, molybdenum,manganese, silicon, titanium and/or copper.

The size of the powder particles may vary within a relatively widerange. Tests have been made with material 5 mm. +0.05 mm. The grain sizeshould be adapted to the demands made on the final product. A coarsepowder results in thicker barriers but a larger distance between thebarriers as compared to a finely divided powder, when using an equalamount of plating metal. The particle size should preferably lie withinthe range 4 +0.1 mm.

In this connection the terms 5 mm. or 4 mm. imply that the materialpasses through a sieve with square sieve openings having a side of 5 or4 mm. respectively.

In this connection the terms +0.05 mm. or +0.1 mm. imply that thematerial stays on a sieve with square sieve openings having a side of0.05 or 0.1 mm. respectively.

In order to obtain dense surrounding layers in the process of platingthe metal particles should not be of too irregular a shape and theyshould have smooth surfaces free from oxide and slag.

In order to obtain a percent material density during the plastic workingthe reduction of the sectional area should be a minimum of 1:4. In thisway one may also be sure that the barriers are given a suitableorientation, i.e. mainly parallel to the surfaces of the sheet metal orof the bar material and consequently mainly angular to the direction ofthe cracks.

By means of the above-mentioned reduction the barrier steel alsoachieves a strength corresponding to that of the basic material. Thesteel produced according to the present invention showed the followingstrength characteristics:

Tensile strength Elongation kg./mm. percent to show how a considerableaccumulation of coating material has taken place at 12. The thickness ofthe barriers may thus vary within a wide range, with considerableaccumulations of plated metal in certain areas and correspondingthinning in others. This process implies that the plating material isnot so fully utilized. The situation may be improved if prior to platingthe particles are formed according to FIG. 3B. The relation D/ d maysuitably be maintained within the range 1.5:1 to 4:1. The grains havebeen flattened in a way to make their shape resemble a disc or anellipsoid.

What we claim is:

1. A process for the production of an austenitic stainless steel alloyhaving a high resistance to transcrystalline stress corrosion in achloride environment afforded by corrosion-impeding barriers oriented inplanes generally parallel to each other comprising the steps of formingon the individual particles of an austenitic stainless steel powder aplated coating virtually covering each particle of a corrosion-resistantmaterial including predominantly nickel, the material being present inthe coated powder in an amount by weight of from about 1% to about basedon the total weight, forming a body of the coated powder, sintering thesaid body, and compacting and permanently deforming the body by plasticWorking substantially to reduce the cross-sectional area of the body ina direction tranusverse to the said parallel planes and substantially toelongate the body in a direction aligned with the said parallel planes.

2. A process according to claim 1 wherein the crosssectional area of thebody is reduced to the extent that the final cross-sectional area is notgreater than about i 8 one-fourth of the original cross-sectional areaof the body.

3. A process according to claim 1 wherein the coating material ispresent in the coated powder in an amount by weight of fromabout 2% toabout 6% based on the total weight. 7

4. A process according to claim 1 wherein the coating material furtherincludes a minor portion of at least one metal selected from the groupconsisting of chromium, molybdenum, silicon, titanium, manganese andcopper.

5. A process according to claim 1 wherein the particles of the stainlesssteel powder pass a screen having square openings measuring 5 mm. andare retained on a screen having square openings measuring 0.05 mm.

References Cited UNITED STATES PATENTS 2,610,118 9/1952 Drapeau 212 X2,933,415 4/1960 Homer 75-212 X 3,223,523 12/1965 Adler 75-212 3,310,8703/1967 Parikh 29420.5

FOREIGN PATENTS 768,067 5/ 1934 France. 38/27,988 9/1963 Japan.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant ExaminerUS. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 II181 Dated November 18 1969 Inventor(s) A. I. Backstrom et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, line 33 "Mineral" should be Material Column 4 line 21, "1.4"should be 1.0

Column 4, line 23 "0. 0" should be 0.3

Column 4 line 24 "0.3" should be 0.4

Column 6 line 64 "5 B" (second occurrence) should be 55 Column 6, line65, "56.2" should be 56.4

SIGNED AN'D SEALED JUN 2 (1970 QEAL) Attest: mum x. soauymm, gm IPatents Edward M. Fletcher, Jr.

Attesting Officer

